The carcinoid syndrome is associated with pathognomonic cardiac lesions which consist of plaque-like fibrous thickenings in mural and valvular endocardium. Ultrastructural study of these plaques in right atrium (2 patients) and tricuspid and pulmonic valve (1 patient) showed similar features. Most cellular elements were mature smooth muscle cells which varied from fusiform to stellate in shape and had greatly thickened, reduplicated basement membranes.